Apple has just started selling DRM-free versions of EMI's music (thanks, Anselm!). And they're including a secret bonus — by watermarking tracks with the identity of the purchaser.
It's fascinating that watermarking, the ugly twin of DRM, has only now made its mass market debut. It should be slightly tricky, although by no means impossible, for users to remove these marks — by converting tracks to a different format at a lower quality, for example, or by writing software to manipulate the audio. It's an extremely difficult problem to design watermarks that are robust against a number of users colluding using multiple copies of the same media content.
Just like cracking DRM, removing watermarks is illegal under the Digital Millennium Copyright Act in the US, the European Union Copyright Directive, and the laws of any other country that has implemented the WIPO Copyright Treaty. However, because watermarks are used to track rather than control media, they have less potential to interfere with users' rights.
Apple seems to be aware of the limitations of watermarks, and using them to track aggregate volumes of audio files uploaded to peer-to-peer networks. They are certainly not strong enough evidence to try suing users (although doubtless we will see the recording industry try).